Solar radiation-based electrical power generating systems typically include:    (a) a receiver that includes a plurality of photovoltaic cells that convert solar energy into electrical energy and an electrical circuit for transferring the electrical energy output of the photovoltaic cells; and    (b) a means for concentrating solar radiation onto the photovoltaic cells of the receiver.
By way of example, the means for concentrating solar radiation may be a dish reflector that includes a parabolic array of mirrors that reflect solar radiation that is incident on a relatively large surface area of the mirrors towards a relatively small surface area of the photovoltaic cells.
In addition to the parabolic array of mirrors, the above-described dish reflector may also include a matched secondary solar radiation modification mirror system (such as a solar flux modifier).
Another, although not the only other, means for concentrating solar radiation is an array of spaced apart mirrors that are positioned to reflect solar radiation that is incident on a relatively large surface area of the mirrors towards a relatively small surface area of the photovoltaic cells.
The present invention relates more particularly, although by no means exclusively, to a large scale solar radiation-based electrical power generating system of the type described above that is capable of producing substantial amounts of electrical power ready for conditioning to at least 20 kW of standard 3 phase 415 volt AC power.
Applications for such large scale power generating systems include remote area power supply for isolated grids, grid-connected power, water pumping, telecommunications, crude oil pumping, water purification, and hydrogen generation.
One significant issue associated with development of commercially viable solar radiation-based electrical power generating systems of the type described above is long term performance of materials and structural integrity of components of the system made from materials as a consequence of:    (a) exposure to extremely high intensity solar radiation capable of producing high temperatures, i.e. temperatures considerably above 1000° C.;    (b) cycling between high and low intensities of solar radiation; and    (c) temperature variations between different parts of structural components.
The receiver is one area of particular importance in this regard.
Specifically, in large scale solar radiation-based electrical power generating systems of the type described above the photovoltaic cells are exposed to solar radiation intensities of at least 200 times the intensity of the Sun during optimum operating conditions. In addition, the photovoltaic cells are subjected to significant cycling between extremely high and low levels of solar radiation and to variations in solar radiation intensity across the surface of the receiver.
An object of the present invention is to provide a receiver that is capable of long term exposure to extremely high intensities of solar radiation, cycling between extremely high and low intensities of solar radiation, and temperature variations between different sections of components of the receiver.
According to the present invention there is provided a system for generating electrical power from solar radiation which includes:    (a) a receiver that includes a plurality of photovoltaic cells for converting solar energy into electrical energy and an electrical circuit for transferring the electrical energy output of the photovoltaic cells; and    (b) a means for concentrating solar radiation onto the receiver; andthe system being characterised in that the receiver includes a plurality of photovoltaic cell modules, each module includes a plurality of photovoltaic cells, each module includes an electrical connection that forms part of the receiver electrical circuit, the receiver includes a coolant circuit for cooling the photovoltaic cells with a coolant, and the coolant circuit includes a coolant flow path in each module that is in thermal contact with the photovoltaic cells so that in use coolant flowing through the flow path cools the cells.
The applicant has found that the above-described receiver is capable of extracting significant amounts of heat generated by incident solar radiation in an efficient and reliable manner. Specifically, the applicant has found that the preferred embodiment of the receiver described in more detail below is capable of extracting up to 50 W/cm2 of exposed photovoltaic cell. Thus, the receiver addresses the significant issue that a large portion of incident radiation on receivers of large scale solar radiation-based electrical power generating systems is not converted to electricity and manifests itself as heat that reduces the efficiency of photovoltaic cells.
In addition, the modularity of the receiver addresses (at least in part) the issue that optimum locations for large scale solar radiation-based electrical power generating systems tend to be in regions that are remote from major population and manufacturing centres and, therefore, construction of the systems in such remote locations presents significant difficulties in terms of transportation of equipment to the sites, on-site construction, and on-going maintenance (including quick replacement of component parts) at the sites.
In addition, the modularity of the receiver makes it possible to enhance manufacture of the receiver because manufacture can be based on repeat manufacture of a relatively large number of relatively small modules rather than a small number of large components.
Preferably in use the coolant maintains the photovoltaic cells at a temperature of no more than 80° C.
More preferably in use the coolant maintains the photovoltaic cells at a temperature of no more than 70° C.
It is preferred particularly that in use the coolant maintains the photovoltaic cells at a temperature of no more than 60° C.
It is preferred more particularly that in use the coolant maintains the photovoltaic cells at a temperature of no more than 40° C.
Preferably each module includes a structure that supports the photovoltaic cells.
Preferably the support structure defines the coolant flow path for extracting heat from the photovoltaic cells.
Preferably the support structure includes:    (a) a coolant member that at least partially defines the flow path, the coolant member being formed from a material that has a high thermal conductivity; and    (b) a substrate interposed between the coolant member and the photovoltaic cells, the substrate including a layer formed from a material that has a high thermal conductivity and is an electrical insulator.
Preferably the coolant member acts as a heat sink.
The coolant member may be formed from any suitable high thermal conductivity material.
By way of example, the coolant member may be a high thermal conductivity metal or ceramic.
Preferably the coolant member is formed from copper.
Preferably the high thermal conductivity/electrical insulator layer of the substrate is formed from a ceramic material.
Preferably the substrate includes a metallised layer interposed between the photovoltaic cells and the high thermal conductivity/electrical insulator layer.
Preferably the substrate includes a metallised layer interposed between the high thermal conductivity/electrical insulator layer and the coolant member.
Preferably the coolant member includes a base, a wall that extends upwardly from the base and contacts the substrate whereby the base, the side wall and the substrate define an enclosed coolant chamber that forms part of the coolant flow path.
Preferably the coolant member includes a series of spaced-apart lands that extend from the base and contact the substrate in a central part of the chamber and define therebetween channels for coolant flow from near one end of the chamber to near an opposite end of the chamber.
Preferably the spaced apart lands are parallel so that the channels are parallel.
With the above-described arrangement there is direct thermal contact between the substrate and coolant flowing through the coolant chamber (including the channels) and between the substrate and the side wall and the lands. This construction provides an effective means for transferring heat from the photovoltaic cells via the substrate to the coolant. In particular, the side wall and the lands provide an effective means of increasing the available contact surface area with the coolant to improve heat transfer to the coolant. This is an important feature given the high levels of heat transfer that are required to maintain the photovoltaic cells at temperatures below 80° C., preferably below 60° C., more preferably below 40° C. A further advantage of the construction is that the side wall and the lands enable lateral movement of the substrate and the coolant member—as is required in many situations to accommodate different thermal expansion of the materials that are used in the construction of the modules. Accommodating different thermal expansion of such materials is an important issue in terms of maintaining long term structural integrity of the modules. In this context, it is important to bear in mind that the high levels of heat transfer that are required to maintain the photovoltaic cells at temperatures below 80° C. place considerable constraints on the materials selection for the components of the modules. As a consequence, preferred materials for different components of the modules and for bonding together different components of the modules are materials that have different thermal expansion. There are two aspects to the issue of materials selection and heat transfer. One aspect is the materials requirements of components of the modules, such as the substrate and the coolant member, to define heat flow paths from the photovoltaic cells to coolant flowing through the coolant chamber. The other aspect is the materials requirements for containing the high hydraulic pressures within the coolant chamber that are required to maintain coolant flow through the coolant chamber at required levels. In particular, the second aspect is concerned with materials selection to achieve sufficient bond strength between the substrate and the coolant member.
Preferably the base includes a coolant inlet and a coolant outlet for supplying coolant to and removing coolant from opposite ends of the chamber, the opposite ends of the chamber forming coolant manifolds.
The above-described coolant inlet, coolant manifolds, coolant outlet, and coolant channels define the coolant flow path of the support structure of the module.
Preferably the ratio of the total width of the channels and the total width of the lands is in the range of 0.5:1 to 1.5:1.
Preferably the ratio of the total width of the channels and the total width of the lands is of the order of 1:1.
Preferably the ratio of the height and the width of each channel is in the range of 1.5:1 to 5:1.
More preferably the ratio of the height and the width of each channel is in the range of 1.5:1 to 2.5:1.
It is preferred particularly the ratio of the height and the width of each channel be of the order of 3:1.
Preferably the receiver includes a frame that supports the modules in an array of the modules.
Preferably the support frame supports the modules so that the photovoltaic cells form an at least substantially continuous surface that is exposed to reflected concentrated solar radiation.
The surface may be flat, curved or stepped in a Fresnel manner.
Preferably the support frame includes a coolant flow path that supplies coolant to the coolant inlets of the modules and removes coolant from the coolant outlets of the modules.
Preferably the coolant is water.
Preferably the water inlet temperature is in the range of 20–30° C.
Preferably the water outlet temperature is in the range of 25–40° C.
Preferably the means for concentrating solar radiation onto the receiver is a dish reflector that includes an array of mirrors for reflecting solar radiation that is incident on the mirrors towards the photovoltaic cells.
Preferably the surface area of the mirrors of the dish reflector that is exposed to solar radiation is substantially greater than the surface area of the photovoltaic cells that is exposed to reflected solar radiation.
According to the present invention there is also provided a photovoltaic cell module for a receiver of a system for generating electrical power from solar radiation, which module includes: a plurality of photovoltaic cells, an electrical connection for transferring the electrical energy output of the photovoltaic cells, and a coolant flow path that is in thermal contact with the photovoltaic cells so that in use coolant flowing through the flow path cools the photovoltaic cells.
Preferred features of the module are as described above.